System V (by AT&T) is the ancestral Unix, and I have used it. But I have never read or heard anything about systems I-IV, other than they existed once upon a time in the dim past. How effective were they, and were they widespread? Is it possible to get one's hand on the code and/or install it? Does anyone here have any experience with these earlier systems?


2 Answers 2


According to wikipedia System V was preceded by System III, a release of UNIX/TS 3. Since there were no official release of UNIX/TS 4, System III was superseded directly by System V.


Earlier versions of Unix were either AT&T internal versions or saw very limited outside use (other than e.g. base for BSD versions, used and mangled under academic licenses; or bases for commercial systems distributed under other names like Microsoft's Xenix).

  • In the context of this question, “earlier versions” would be System III, and that did see quite a lot of use outside of AT&T. Mar 2, 2020 at 10:21

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